Other forms: gobemouches
A gobemouche is the opposite of a skeptic, a person so innocent that they trust what they're told even when they shouldn't.
Gobemouche comes from two French words meaning "to swallow whole" and "fly," as in the insect. So the whole word literally means "fly-swallower." Imagine a naive, trusting person standing around with their mouth open and you'll get the idea. It's also pretty common to say that someone "swallowed" a lie as a way to mean that they believed it, so you could also think of a gobemouche as someone who will swallow anything they're told, even if it's a lie — or a fly.